MAPT and tauopathy: Specifically, a circular bifunctional circDNA aptamer was synthesized from 1) a transferrin receptor (TfR)-targeting aptamer that facilitates TfR-induced transcytosis across the endothelial cells in the blood-brain barrier (BBB), and 2) a Tau protein-specific aptamer that inhibits Tau phosphorylation and tauopathy-related pathological events in the brain.